1. Field of the Invention
This invention relates to a process for intensifying photographic silver images by physical development, and to an improvement in a physical developer for use in this process. More specifically, it relates to a process for reproducing photographic images of increased optical density from photographically formed silver images by physical development (intensification) using non-silver metals.
2. Description of the Prior Art
The developing operation, which plays an important role in processing silver halide photographic materials to form images thereon, embraces chemical development and physical development both of which are well known.
In chemical development, a silver halide in a photographic material is exposed and subjected to the action of a developer solution comprising a developing agent for reducing the silver halide, such as Metol (tradename for p-methylaminophenol sulfate) and hydroquinone, and developing aids, whereby the silver halide is reduced to silver and deposits mainly where the latent image formed by exposure exists in the silver halide crystals.
Physical development, on the other hand, comprises treating a nucleus (latent-image silver) or print-out image silver formed by exposure (these types of silver will be inclusively referred to hereinbelow as "image-forming silver") with a developer solution containing a silver ion and a reducing agent thereby to deposit silver on the nucleus and intensify it. Hence, in physical development, the image-forming silver is supplied from the silver ion in the developer solution. A diffusion transfer development method whereby silver halide in the photographic layer is first converted to a complex salt and dissolved, and then reduced is sometimes classified as physical development.
The chemical development method has been the predominant technique used in the processing of silver halide photographic materials in general, and has gained widespread acceptance because it can give rise to superior characteristics such as sensitivity, tone reproduction, or a well balanced combination of resolution and sensitivity in photographic materials. In contrast, the physical development method has not been commercially accepted to any substantial extent except in specialized applications, because it is inferior to chemical development in a number of respects in spite of its ability to provide ultrafine graininess, high resolution, and high covering power. For example, physical development results only in low sensitivity and requires a long developing time, and moreover, physical developer solutions are chemicaly unstable.
However, both chemical development and physical development have the common disadvantage that the silver used to form images is very costly. In an attempt to remove this disadvantage, extension investigations have been performed in recent years for making images of the desired photographic characteristics by using lesser amounts of silver in chemical development. Investigations have also been conducted on improved methods of physical development wherein the amount of silver in the photographic layer of a photographic material used is reduced, and a silver image is first formed although in a low density, after which the low density silver image is intensified by treatment with a physical developer solution containing a metal more base than silver (i.e., a metal having an ionization tendency higher than that of silver hereinafter for brevity a "metal more base than silver" or "non-silver metal"), such as cobalt, nickel or copper, thereby depositing the metal on the silver image and affording a metal image of the desired optical density. For example, U.S. Pat. No. 3,650,748 discloses a method which comprises chemically developing a silver halide photographic material, subjecting the developed material to an activating treatment, and then physically developing it with a metal more base than silver, such as cobalt, nickel or copper. Various other types of physical development methods are described, for example, in U.S. Pat. Nos. 2,532,284, 2,690,401, 2,690,402, 2,726,969, 2,762,733, 2,871,142, and 3,011,920, French Pat. Nos. 2,938,805, 1,225,194 and 1,182,849, and Belgian Pat. No. 637,058. All of these methods, however, require an activating treatment (pretreatment). In using nickel or cobalt, the photographic material is treated with an acid solution of stannous chloride and further with a palladium salt. Treatment with silver nitrate, for example, is necessary in using copper. This activating treatment is for the purpose of treating the silver image with a metal catalyst, for example a solution of a noble metal salt such as a palladium, gold or platinum salt to deposit a nucleus of the metal on the surface of the silver image, and imparting a catalytic ability to the silver image for subsequent physical development.
Such a physical development process is being exploited as one characteristic process of silver halide photographic processes. The process would be more advantageous if a material which has the same function as a silver nucleus can be directly produced by the action of light or development at the time of physically developing the photographic layer. In other words, an imaging process combining chemical development of silver halide and the physical development of a non-silver metal is desirable since the amplification action in the step of rendering the image visible is twofold and the treatment is simple. Hence, the advent of such an imaging process has long been desired in the art.
According to the prior techniques, a developed silver image obtained by subjecting a silver halide photographic material to a series of exposure, development and fixation, or a nucleus (latent-image silver) or print-out silver image obtained by light exposure and fixation cannot be directly intensified by physical development with a non-silver metal (chemical plating by a non-silver metal). Known reducing agents used in the conventional physical developers containing non-silver metals include, for example, formaldehyde, hypophosphites, borohydride compounds, hydrazine compounds, and amine borane compounds. However, reducing agents which cause the direct deposition of non-silver metals on the silver nucleus or developed silver formed from silver halide photographic material, and physical developer solution containing such reducing agents have not been known heretofore.
U.S. Pat. No. 3,650,748 discloses that a chemically developed silver is inert to physical development, and cannot be a nucleus for non-silver metal physical development.
In other words, this patent teaches that a metal more base than silver does not deposit in the region of developed exposed silver. Such silver is not a catalyst for reduction in physical development. This patent discloses a process for image reproduction by physical development using a metal more base than silver, which comprises activating such silver with a salt of a metal more noble than silver (e.g., platinum, palladium or gold), a mineral acid such as nitric acid, hydrochloric acid, sulfuric acid or sulfurous acid, a strong oxidizing agent such as potassium ferricyanide or hydrogen peroxide, or a reducing agent such as sodium borohydride thereby to form an active nucleus on the surface of or surrounding the silver image; and then contacting the photographic material containing this silver image with a physical developer solution containing a salt of a metal such as copper, nickel, cobalt or iron and an amine borane reducing agent. The purpose of activation is to deposit copper, nickel, cobalt, iron or the like selectively on the silver image. In other words, since the photographically formed developed silver is inert to physical development with a salt of a metal more base than silver a catalytic nucleus is required. The catalytic nucleus is provided by forming an active nucleus on the surface of or surrounding the silver image.